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Holmes' attorneys say they are still exploring a mental health defense

Colorado shooting suspect James Holmes has offered to plead guilty and spend the rest of his life behind bars in exchange for avoiding the death penalty, according to court documents filed Wednesday.

Prosecutors have not accepted the offer. Previously, they had said they would make a decision on whether to seek the death penalty against the 25-year-old Holmes at a hearing April 1.

"Prior to arraignment, Mr. Holmes made an offer to the prosecution to resolve this case by pleading guilty and spending the rest of his life in prison, without any opportunity for parole," the documents read.

"If the prosecution elects not to pursue the death penalty, then it is Mr. Holmes' position that this case could be resolved on April 1."

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In the documents filed Wednesday, his attorneys said they are still exploring a mental health defense, "and counsel will vigorously present and argue any and all appropriate defenses at a trial or sentencing proceeding, as necessary."

"Nevertheless, Mr. Holmes is currently willing to resolve the case to bring the proceedings to a speedy and definite conclusion for all involved," they said.

Holmes is charged with a total of 166 counts of murder, attempted murder and other charges.

Federal agents have said the former University of Colorado doctoral student planned the attack for months.

He began buying guns in May 2012, allegedly building an arsenal of two Glock handguns, an AR-15 rifle, a shotgun and 6,295 rounds of ammunition.

Authorities say Holmes booby-trapped his apartment with explosives, then traveled to the movie theater armed with the weapons, tear gas and body armor planning to kill audience members during a screening of "Batman: The Dark Knight Rises."

Witnesses who spoke to CNN said the gunman roamed the theater, shooting randomly as people tried to scramble away or cowered between seats.

"This is not a 'whodunit.' Everybody knows that James Holmes committed these horrible murders. The question is what punishment he will get," said CNN senior legal analyst Jeffrey Toobin.

"If they (defense attorneys) can get life in prison, they will consider this a victory."